The field of the disclosure relates generally to control of rod pumping units and, more particularly, to a control system and a method of controlling a linear pumping unit as a function of load and position.
Most known rod pumping units (also known as surface pumping units) are used in wells to induce fluid flow, for example oil and water. The linear pumping unit is a particular type of rod pumping unit that converts rotating motion from a prime mover (e.g., an engine or an electric motor) into reciprocating motion above the wellhead. This motion is in turn used to drive a reciprocating down-hole pump via connection through a sucker rod string. The sucker rod string, which can extend miles in length, transmits the reciprocating motion from the wellhead at the surface to a subterranean piston and valves in a fluid bearing zone of the well. The reciprocating motion of the piston valves induces the fluid to flow up the length of the sucker rod string to the wellhead.
Rod pumping units are exposed to a wide range of conditions. These vary by well application, the type and proportions of the pumping unit's linkage mechanism, and the conditions of the well. Furthermore, well conditions, such as downhole pressure, may change over time. These conditions may cause variability in the flow of the fluid. In addition, these conditions affect the sucker rod string. The sucker rod string transmits dynamic loads from the down-hole pump to the rod pumping unit. The sucker rod string behaves similarly to a spring over long distances. The sucker rod string elongates and retracts based on exposure to variable tensile stress. The response of the sucker rod string is damped somewhat due to its submergence in a viscous fluid (water and oil), but the motion profile of the rod pumping unit combined with the step function loading of the pump generally leaves little time for the oscillations to decay before the next perturbation is encountered.
The rod pumping unit imparts continually varying motion on the sucker rod string. The sucker rod string responds to the varying motion by sending variable stress waves down its length to alter its own motion. The sucker rod string stretches and retracts as it builds the force necessary to move the down-hole pump and fluid. Traveling stress waves from multiple sources interfere with each other along the sucker rod string (some constructively, others destructively) as they traverse its length and reflect load variations back to the rod pumping unit, where they can be measured. The various stresses on the sucker rod string cause fatigue on the rods, ultimately impacting the reliability of the rod pumping system. While this problem is widely recognized, commercially available rod pump controllers fail to mitigate it, and typically are controlled to maintain fluid fillage and tubing reliability only by speeding up or slowing down the pumping unit with a single velocity profile.